For many years various methods of spraying paint by means of jets of compressed air impinging against a cylindrical or conical, hollow or solid, jet of paint have already been proposed in the art.
Methods of spraying paint by using the hydrostatic pressure, which are frequently referred to as "without compressed air" methods, are already known.
The inconveniences of pneumatic atomization lie essentially in the relatively high compressed-air consumption and also in the strong jet of air and atomized paint sprayed through the nozzle, so that a substantial amount of paint not deposited on the object is lost in the surrounding atmosphere while forming many eddies.
Hydrostatic spraying is objectionable in that it requires relatively high paint pressures and therefore very small atomizing orifices that are difficult to machine and exposed in service to a considerable wear by abrasion; furthermore, these small orifices are liable to clog up rapidly. On the other hand, the energy contained in the high-pressure paint jet is relatively high and causes the jet of atomized paint to travel at a very high speed, thus creating an induced air stream leading to appreciable losses of paint.
Under these conditions, various attempts have been made with a view to reduce the hydrostatic pressure, but this trend was rapidly stopped when it occurred that the jet deteriorated by allowing two jet portions to escape along its edges, these two portions being more or less separated by a central portion. Now these lateral jet portions are not properly atomized and produce two highly objectionable paint concentrations.
Various attempts for improving the hydrostatic atomization, notably by modifying the shape of the projection device, did not yield really significant results.
However, better results were obtained by modifying the paint jet issuing from the hydrostatic atomization nozzle by using jets of compressed air or, still better, by causing this paint jet to be carried along by a pair of parallel air jets of same direction and shape as, and surrounding and holding, the paint jet. Thus, the proper shape of the hydrostatically atomized jet is preserved and the atomization is slightly improved, and a hydrostatic atomization improved by the compressed air affording the use of lower paint pressures and also of larger jet orifices was obtained.